Storm-Dominated Cyclic Shelf Deposits in Upper Ordovician of Cincinnati Arch Region
David C. Jennette, Wayne A. Pryor
The Kope and Fairview Formations and the Bellevue Toungue of the Grant Lake Limestone (Upper Ordovician, Cincinnatian Series) exposed in the Cincinnati, Ohio, region represent storm-dominated regressive sedimentation on an intracratonic ramp or shelf. The Kope-Fairview transition zone (25 m) is an outer shelf lithofacies composed of numerous small-scale (1-3m), shallowing-upward cycles. These cycles consist of two facies:(1) a shale-dominated component characterized by numerous, single-event distal tempestites, and (2) a fossiliferous limestone component characterized by amalgamated, proximal tempestites. Distal storm components contain abundant thin-bedded, parautochthonous packstones, rare autochthonous fossil accumulations, and abundant thin-bedded siltstones that comm nly contain tool marks, gutter casts, parallel lamination, small-scale hummocky-cross-stratification, hummocky bed forms, mud-filled scours, and millimeter ripples. Proximal tempestites are thick-bedded, fine to coarse-grained packstones and grainstones that commonly have megarippled surfaces, cross-lamination, and shale lithoclasts, and contain a mixture of fossils showing degrees of abrasion and transport. Lower Fairview cycles are proximal equivalents of underlying Kope cycles, and typically show more scour features, bedding amalgamation, and proximal components. Cycles are laterally continuous across the study area (25 × 15 mi) and exhibit only minor variation. Cycle boundaries appear isochronous and mark the abrupt transition from proximal to distal storm sedimentation. The wid spread character and isochronous relationship suggest that these cycles are the product of an allocyclic mechanism involving episodic, but rapid, rises in relative sea level. Deposition of cyclic sequences occurred on a very gently dipping, northward-facing shelf. Paleocurrent analyses indicate that storm currents flowed northward, perpendicular to isobaths, in the offshore direction.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.